Existing systems use a conventional piston pump to dose additive liquid where it has to be injected (mostly in the engine, the exhaust gases, the fuel tank, the fuel feed pipe or the fuel return pipe of said engine; this will generally be called “engine circuit” in the present application). This piston pump is a fixed volume pump, where the pump doses a constant pre-set volume per stroke (generally depending on the volume of the cylinder in which the piston moves).
US Patent Application 2003/0136355 discloses a system for feeding a fuel additive inside a fuel tank by means of a metering pump which is integrated in the fuel-drawing module. This metering pump is a positive displacement pump (piston pump) delivering a constant volume of additive on each pump cycle. The total required dose is delivered by actuating the pump the required number of cycles. The disadvantages are:                that any error associated with the piston volume is accumulated over the number of strokes, while an accurate dose is required for optimal results;        possible overdosing (because of the incapacity of delivering fractions of the cylinder volume), which is expensive considering the price of the additive; and        that the solenoid valves used for the actuation are typically noisy.        
The new invention aims at resolving at least some of these disadvantages.